Exhausting exhaust

The basic ventilation questions are answered and most of the duct work is installed, with one exception. And that exception is the exhaust duct from the energy recovery ventilator (ERV) to the roof.

Our mechanical engineer placed the fresh air intakes on the east side of the building and routed the exhaust through the closets to and through the roof. I learned during the plan review process that the City of Chicago insists that there is “no communication between the outside air [fresh air intake] and exhaust air”.

In other words, having the fresh air intakes located on the east building elevation and the exhaust on the roof does not only make sense but was a good move. That said, I had a hard time warming up to the idea of running the exhaust duct through the closet spaces and out the roof.

I am sitting on a chimney with three flues that have “use me!” written all over them. We re-used one flue by routing the radon removal system through it. I thought the other two flues should be set aside to accommodate the ERV exhaust. All this will save me potentially troublesome roof penetrations.


Great idea! But things ain’t that simple.

Because the exhaust air is likely to carry a lot of moisture, I’ll need to line the chimney with some kind of metal duct. And this was the part I could not figure out. What kind of duct should I use and how should I deal with the connections?

I racked my brain over this for weeks.  To make a long story short, I eventually called on Mariusz again for help. He is our heating and plumbing contractor. Once I walked him through my problem, he reassured me with his most favored sentence: “Marcus, this is easy!” And it was.

I had solved a similar problem once already, when I installed the radon stack.

We assembled the ducts on the roof and sealed the seams and all connections with duct mastic. The first section was an eight inch duct with a 90 degree elbow at the bottom. That elbow connects through the chimney wall into the basement living space.

The next duct section we connected was increased to 10 inches. We now began to carefully lower the duct into the chimney flue, and continued to add section after section until we reached the connection point in the flue at the garden unit level.


We connected the duct work at the bottom, added the flanges and a cap at the top, placed and sealed the chimney cap … and all this took less than one hour!

Why did we increase the duct size from eight to 10 inches? Because I would like to connect the 2nd floor ERV exhaust to the same duct. The exhaust air flow from the two ERV’s (garden unit and 2nd floor) requires a duct size of 10 inches.

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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